The invention relates to a method of straightening a skew in a cut sheet and an apparatus to which such method is applied.
In a sheet feed unit for feeding piled cut sheets toward a print section or the like on a single sheet basis, widthwise displacement of the cut sheet at an initial stage of sheet feeding leads to skew feeding, which causes the problem of wrinkling or jamming the sheet thereafter.
To overcome this problem, an apparatus is disclosed in U.S. Pat. No. 4,248,415, which operates as follows. A sheet fed by a sheet feed roller is suspended by a pair of sheet forward rollers that are rotating backwardly on a downstream side in a sheet forward direction of the sheet feed roller, so that the position of a front end of the sheet can be corrected by reaction derived from the flexion of the sheet which is caused by such suspension. Further, an apparatus proposed in Japanese Patent Examined Publication No. 62-38261 is designed to correct the position of the front end of the sheet by utilizing reaction derived from the flexion of the sheet which is caused by discharging the sheet while reversely rotating a sheet forward roller when the front end of the sheet has been nipped by the sheet forward roller completely.
These apparatuses are simple in design in that the hardness of a sheet is utilized to correct the position of the sheet. However, both apparatuses entail much time in straightening a skew in a cut sheet because they require that the sheet be flexed. In addition, these apparatuses are not sufficiently effective in straightening the skew because the reaction is small even if the sheet is flexed in the case of a soft sheet, and because the sheet cannot be flexed at all in the case of a hard sheet.
The invention has been made in view of the above circumstances. Accordingly, the object of the invention is to provide a novel skew straightening method and a apparatus therefor, which ensure a quickest operation and which are effective, reliable, and applicable to any kind of paper, either hard or soft.